The disclosure relates to a method for obtaining an indication, in particular an initial indication, of a possible faulty load condition of a multi-phase electric motor.
Faulty load conditions in electric motors are critical particularly when short circuits occur. Such short circuits are normally detected on the basis of voltage drops along the drain-source paths of the internal or external driver transistors of the drive unit that are used to drive the electric motors or other loads. Normally, these voltage drops are evaluated individually, i.e. per phase of the motor. In case that a metrologically detected current is higher than the maximum current allowable in operation, this is an indicator of an external short circuit in at least one of the motor connection lines. A disadvantage in the detection of short circuits resides in that particularly “soft” short circuits toward ground cannot be detected in all cases. A further problem consists in that, in case of relatively high-ohmic short circuits or short circuits with added inductive factor of the short circuit current, the overcurrent will partly not lie outside the maximum current allowable in operation of the motor. Further, in added inductive factors and in PWM controlling of loads such as e.g. electric motors, it may happen that the increase of the current in case of a short circuit is so slow (“soft” short circuit) that a PWM cycle will have ended before the overcurrent switch-off threshold value has been reached. Nonetheless, in such cases, the power transistors may heat up so massively that damage will be caused that can occur so fast that also a detection of excess temperature, due to its temporal delay, will not offer sufficient protection.
From DE-A-10 2011 004 911, DE-B-10 2004 009 046, DE-A-102 36 377 and U.S. Pat. No. 5,266,891, methods for detection of faulty load conditions in multi-phase electric motors are known, wherein, according to each of said documents, the temporal development of the phase currents is evaluated.